The present invention relates to microwave absorbers used in RF anechoic chambers or the like, and a method of manufacturing the same, and more particularly, to a semi-incombustible or incombustible microwave absorber excellent in its microwave absorbing capacity, and a process for manufacturing the same.
Hitherto, the RF anechoic chamber has been widely employed for uses of measurement of antenna characteristic, and measurement of radio noise emission. In such RF anechoic chambers, to eliminate reflection of microwaves in the chamber, as shown in FIG. 10, the walls of an electromagnetic shielding enclosure 3 are lined with microwave absorbers 4.
The microwave absorber 4 is to absorb incoming microwave without reflection and convert into heat. In an ordinary microwave absorber 4, a resin foam impregnated with carbon black or the like is used. It is often formed in a pyramid-shape as shown in FIGS. 11(a) and 11(b). Although the microwave absorption characteristic in high frequency region is inferior to that of pyramid form, as shown in FIGS. 12(a) and 12(b), a wedge-shaped microwave absorber 4' is also known. FIGS. 11(a) and 11(b) show side view and plan view of the pyramid-shape microwave absorber 4, and FIGS. 12(a) and 12(b) show similarly side view and plan view of the wedge-shape microwave absorber 4'.
As the resin foam, usually, foamed polystyrene, foamed polyurethane, or foamed crosslinked polyethylene is used.
In forming foamed polystyrene, beads of foamable polystyrene are used. These beads are, however, relatively large particles of about 0.1 to 1 mm. Therefore the cell diameter of the foam becomes large, and a sufficient microwave absorption is not achieved unless much carbon black is mixed in. Moreover, the foamed polystyrene is limited to the use of relatively low frequency band, and cannot be used for high frequency bands of, for example, 10 GHz or more. Furthermore, since the cut foamed polystyrene is fragile, the tip of the pyramid-shape microwave absorber is likely to be broken, and hence a particular caution is needed.
On the other hand, foamed polyurethane is soft, so that the tip is hardly broken if formed in a pyramid-shape, and the cell diameter is small, so that it can be used in high frequency band of 10 to 100 GHz. However, to manufacture a microwave absorber of polyurethane foam requires the steps of jigging the foam, immersing the foam in a latex liquid containing carbon black in a compressed state, releasing the compression to impregnate the latex, and drying. Therefore, the impregnated latex is moved to the lower part when drying, so that the carbon black cannot be contained uniformly, and unevenness is likely to occur.
Because of use of foamed polyurethane, moreover, only a flame retardant microwave absorber can be obtained. However, when a strong microwave is continuously radiated to a same position of the microwave absorber, heat is generated inside due to dielectric loss, which may lead to risk of ignition or combustion. Accordingly, the flame retardant material is insufficient, and semi-incombustible or incombustible material is required.
On the other hand, as for crosslinked polyethylene foam, only a flame retardant material can be manufactured, the same as in the case of foamed polyurethane, and semi-incombustible or incombustible material can not be obtained. In particular, the crosslinked polyethylene foam is easily melted by heat, and hot molten lumps may drop to burn the people standing beneath the absorber.
Besides, since the microwave absorbers using such foams contain black conductive powder (dielectric loss material) such as carbon black, and the surface has a dark gray or black color, therefore when installed in RF anechoic chambers or the like, the lighting effect is low, and it may not only look dark, but also give a threatening feel to the user because of the color and pointed shape of pyramid or the like. Conventionally, therefore, the surface of the microwave absorber was painted in blue or the like after forming, which took labor, and uniform painting was difficult because of the pointed shape, and it was hence costly.